Most successful vaccines have been developed empirically, with little or no immunological insights. Continued existence of this gap has been a major barrier to the successful development of rationally designed vaccines. Due to their capacity to elicit and regulate immune responses, dendritic cells (DCs) are essential to any effort for vaccine development. The study of conventional DCs (cDCs) from humans has focused on cells in blood, which comprise BDCA-1+ myeloid DCs, BDCA-2+ plasmacytoid DCs, and a small subset of BDCA-3 high, myeloid DCs. The heterogeneity among DCs is of interest because of the specialized functional properties of each DC subset. But a comprehensive model that describes cDC genesis and the origins of cDC functional diversity in humans is lacking. To address this important gap in our understanding of this essential cell type, we propose to uncover the origin of human cDCs and to determine how adjuvants or vaccines alter cDC development. The hypothesis to be tested is that human cDC development, subset distribution, and their state of activation is altered by administration of adjuvants or vaccines. We will first elucidate the origin of cDCs in humans by identifying the human pre-DCs and by determining whether there are one or two lineages of pre-DCs corresponding to the two major myeloid cDC subsets (aim 1). Second, using clinical samples from individuals treated with increasing doses of Flt3L, we will determine if FItSL administration might Improve vaccine efficacy by mobilizing cDCs and their progenitors (aim 2). Lastly, we will define if adjuvants (I.e. TLR3 and TLR4 agonists) or vaccines (I.e. FDA-approved vaccines as well as a DC-targeted vaccine) administration impacts on cDC development in humans (aim 3). Taken together, these experiments should provide a comprehensive model of cDC development both in steady state and inflammatory condition. Understanding these processes will provide important insights into vaccine design.